Recent published evidence confirms that at least some trilobites reproduced using eggs, apparently stored in or expelled from the cephalon. However, it has not been determined whether trilobite eggs hatched internally or were extruded, either discretely or in masses. A ~6 cm2
area on a claystone specimen from the Upper Cambrian (Paibian) Conasauga Formation in western Georgia, contains a cluster of >100, mostly complete, meraspid trilobites identified as juvenile Aphelaspis
sp. The juvenile trilobites, all approximately 2.0 mm total length, have their long axes apparently randomly aligned, indicating that their association is not the result of current sorting. They appear to be of largely uniform meraspid degree with five free thoracic segments, suggesting that they hatched penecontemporaneously from a single egg deposition event. Further, they are on a single plane within the small area, suggesting they either remained in place on the egg-deposition site, or that they selectively massed together post-hatch as protective behavior.
Gregarious behavior by trilobites (aka “trilobite clusters”) has been abundantly reported, but encompassing only holaspid adults; this is the first occurrence of juvenile trilobite clustering, and one of the few involving clusters of Cambrian trilobites. Numerous explanations for clustering behavior have been posited, but few have indicated that clusters follow egg deposition as suggested by the present specimen.